Process for supplying a gaseous product to meet a fluctuating demand



' Oct. 16, 1962 P. M. SCHUFTAN 3, 1 PROCESS FOR SUPPLYING A GASEOUS PRODUCT TO MEET A FLUCTUATING DEMAND 'Filed Dec. 15, 1960 3,058,315 PRGCESS FOR SUPPLYKNG A GASEOUS PRODUCT Ti) MEET A FLUCTUATMG DEMAND Paul Maurice Schuftan, Richmond, England, assignor to The British @xygen fiompany Limited, a British com- P y Filed Dec. 13, 196%, Ser. No. 75,580 Claims priority, application Great Britain Dec. 14, 1959 8 Ciaims. (Cl. 6252) This invention relates to the low temperature separation of gas mixtures and more particularly to a method of satisfying a fluctuating or intermittent demand for a separation product in gaseous form.

It is well known that the operation of plants for the separation of gas mixtures at low temperatures becomes uneconomical if the consumption rate for a gaseous product is fluctuating or intermittent.

One way of Coping with such conditions is to design the separation unit for the peak demand rate, which actually may greatly exceed the average demand, and to reduce the load at periods of low demand or to stop the plant altogether at periods of no demand. This involves a large and expensive separation plant and in view of its unsteady loading its operation is ditficult and bound to be ineflicient.

Another method is to design the separation plant for the average demand rate, in which case adequate storage capacity for the product produced during periods of low or no demand must be provided, which usually proves to be very costly.

It is an object of the present invention to provide a method of operating a gas separation plant to satisfy a fluctuating or intermittent demand for a gaseous product which does not suffer from these disadvantages.

According to the present invention, a method of operating a low-temperature gas separation plant to satisfy a fluctuating or intermittent demand for a gaseous separation product comprises operating the plant continuously at the average demand rate for the product, condensing at least a part of the product produced during periods of no or reduced demand by heat exchange with a stored liquefied auxiliary fluid vaporising at a temperature below the condensation temperature of the product, storing the condensed product, revaporising sufiicient of the stored condensed product at a rate adequate to satisfy the excess demand during periods of high demand by means of a stream of the auxiliary fluid in the gaseous state and at a pressure higher than that at which this fluid is vaporised to condense the produce during periods of no or reduced demand, and storing the auxiliary fluid which is liquefied during the revaporisation for use in condensing said product during a subsequent period of no or reduced demand. The auxiliary fluid is preferably another separation product.

This method of operation enables the separation plant to operate continuously at the average demand rate for the product, whilst excess production during periods of no or reduced demand is stored as liquid, and thus does not require the provision of extensive and costly storage capacity. The additional equipment required consists essentially of tanks for the storage of the liquefied product and the liquefied auxiliary fluid, a compressor and heat exchangers for the auxiliary fluid, and a liquefier for the product.

As an example the invention will now be described for the case of an air separation plant which has to supply a gaseous oxygen product for a period of eight hours at a rate of 300 tons/day, alternating with four hours of no demand.

According to the invention the capacity of the air separation plant is required to correspond to the average Patented Oct. 16, 1962 "ice demand only, i.e. 200 tons/ day, and the plant will be kept in continuous operation.

During the four hours of no demand a quantity of 200 X =33% tons of oxygen has to be liquefied at a rate of 200 tons/day, while during the eight hour period of demand the 33 /3 tons has to be revaporised at a rate of tons/day.

If the air separation plant produces simultaneously with the oxygen an adequate quantity of dry nitrogen, this can be used as the auxiliary heat transfer fluid.

Alternatively, the nitrogen can be supplemented with air or air only can be used as auxiliary fluid. Drying of air and removal of carbon dioxide therefrom may then become necessary under certain conditions.

At the start of operations an initial quantity of about 33 /3 tons of liquid oxygen has to be made available for the case under consideration, either produced from the air separation plant or imported from an external source. During the eight hour period of demand this will be vaporised in a heat exchanger system by an equivalent quantity of compressed nitrogen which liquefies during this operation and is stored.

The vaporised oxygen is admitted with the oxygen produced at constant rate in the air separation plant.

During the four hours of no demand this liquefied nitrogen is used to condense in another heat exchanger 33 /3 tons of gaseous oxygen which is stored in a liquid oxygen tank.

The pressure at which the nitrogen is required for vaporising the liquid oxygen depends on the pressure at which the oxygen is required. If the oxygen is produced or required under pressure the vaporisation of the liquid nitrogen could, if desired, be carried out at elevated pressure, but in all cases the condensing pressure of the nitrogen or other auxiliary fluid has to exceed its vaporisation pressure.

If storage of the liquid oxygen and liquid nitrogen is preferred at atmospheric pressure, while the oxygen is produced or required at a higher pressure, liquid pumps at the outlets of the tanks could bring the liquid oxygen, and if required also the liquid nitrogen, to the required pressures. In this case and in order to recover the flash loss caused by the expansion of the liquefied high pressure nitrogen to the lower storage pressure, a high pressure/low pressure heat exchanger is preferably provided in parallel to the liquid oxygen vaporiser.

Should the cold produced by the expansion of the compressed auxiliary fluid to the storage or vaporisation pressure not be sufficient to cover the vaporisation losses from the liquid storage tanks, the extra cold required can be supplied from the separation unit, for example, by a continuous production of a small fraction of the oxygen in the liquid state.

While in the foregoing example the invention has been described for the most difficult case of intermittent demand, it is equally suitable for fluctuating demand around a basic load when the excess or lack of production can be automatically compensated by liquefaction or vaporisation of the product.

The invention is not restricted to the case of air separation which has just been described. It could be similarly applied, for instance, to a plant producing simultaneously ethylene and methane and in which the demand for ethylene is fluctuating or intermittent. In this case methane would be a suitable auxiliary fluid.

One embodiment of the invention will now be described with reference to the accompanying drawing which shows a flowsheet of an air separation plant adapted to operate a process according to the invention.

The air separation unit indicated at 1 is of conven- 3'- tional construction and produces a gaseous oxygen product which leaves the unit through a line 2. Waste nitrogen leaves the unit through a line 3.

The unit 1 is operated continuously so as to provide the gaseous oxygen at the average demand rate. When there is no or a reduced demand for oxygen, the excess oxygen produced is diverted through a branch line 4 from the line 2 into a liquefier 5 where it is condensed by heat exchange with liquid nitrogen withdrawn from a storage vessel 6 through a line 7 controlled by a valve 7a. The nitrogen so vaporised passes to waste through a line 8 or is returned to line 3. The liquid oxygen so produced is stored in a storage vessel 9.

During periods when the demand for oxygen is greater than the average, the oxygen produced by the air separation unit 1 is supplemented by gaseous oxygen produced by vaporising the liquid oxygen stored in the vessel 9. The liquid oxygen is withdrawn through a line 10 and forced by a pump 11 through a vaporiser 12 in which it is vaporised by heat exchange with compressed gaseous nitrogen as hereinafter described. The oxygen so vaporised is fed to the line 2 through a line 13.

A part of the dry gaseous nitrogen fraction leaving the air separation unit 1 through the line 3 is withdrawn through a branch line 14, compressed in a compressor 15 and thereafter divided into two streams. The major stream is passed through the vaporiser 12 Where it serves to vaporise the stored liquid oxygen and is itself liquefied. The liquefied nitrogen is expanded through a valve 16 into a separator 17, in which nitrogen vapour formed during the expansion is separated from the liquid. The vapour is then passed through a heat exchanger 18 in heat exchange with the minor stream of compressed nitrogen which is thereafter expanded through a valve 19' into the separator 17. The low pressure nitrogen vapour leaving the exchanger 18 is returned through line 20 to line 14 upstream the compressor 15.

As an alternative and in order to save power, separator 17 can be operated at an intermediate pressure and the nitrogen vapour is then returned to a higher stage of the nitrogen compressor.

The liquid nitrogen from the separator 17 is passed through a line 21 controlled by a valve 22 to the storage vessel 6, where it is stored for use in condensing oxygen during a subsequent period of no or reduced demand.

I claim:

1. A method of operating a low-temperature gas separation plant to satisfy a fluctuating or intermittent demand for a gaseous separation product comprising operating the plant continuously to produce said gaseous separation product at the average demand rate therefor, condensing at least a part of said product produced during periods of no or reduced demand by heat exchange externally of said plant with a stored liquefied auxiliary fluid vaporising at a temperature below the condensation temperature of said product, storing the condensed product, revaporising suflicient of the stored condensed product during periods of high demand at a rate adequate to satisfy the excess demand by heat exchange with a stream of said auxiliary fluid in the gaseous state and at a pressure higher than that at which this fluid is vaporised to condense the product during periods of no or reduced demand, and storing the auxiliary fluid which is liquefled during the revaporisation for use in condensing said product during a subsequent period of no or reduced demand.

2. Method according to claim 1 wherein said auxiliary fluid is another separation product.

3. Method according to claim 1 wherein said gaseous product is produced or required at elevated pressure whilst said liquefied product is stored at atmospheric pressure and is pumped to the required elevated pressure before revaporisation.

4. Method according to claim 1 wherein said auxiliary fluid is expanded after liquefaction to a pressure intermediate its initial pressure and that to which it is compressed prior to liquefaction.

5. Method of operating a low-temperature air separation plant to satisfy a fluctuating or intermittent demand for gaseous oxygen, comprising operating the plant continuously to produce gaseous oxygen at the average demand rate therefor, condensing at least a part of the gaseous oxygen produced during periods of no or reduced demand by heat exchange externally of said plant with a stored liquefied auxiliary fluid vaporising at a temperature below the condensation temperature of the oxygen, storing the condensed oxygen, revaporising suflicient of the stored liquid oxygen during periods of high demand at a rate adequate to satisfy the excess demand by heat exchange with a stream of said auxiliary fluid in the gaseous state and at a pressure higher than that at which this fluid is vaporised to condense oxygen during periods of no or reduced demand and storing the auxiliary fluid which is liquefied during the revaporisation for use in condensing oxygen produced during a subsequent period of no or reduced demand.

6. Method according to claim 5 wherein said auxiliary fluid is nitrogen, or air, or a mixture of nitrogen and air.

7. Method according to claim 5 wherein the oxygen is produced or required at elevated pressure whilst said liquefied oxygen is stored at atmospheric pressure and is pumped to the required elevated pressure before vaporisation.

8. Method according to claim 5 wherein said auxiliary fluid is expanded after liquefaction to a pressure intermediate its initial pressure and that to which it is compressed prior to liquefaction.

References ited in the tile of this patent UNITED STATES PATENTS 2,685,181 Schlitt Aug. 3, 1954 2,788,646 Rice Apr. 16, 1957 FOREIGN PATENTS 798.584 Great Britain July 23, 

